Electron emissive cathode



INVENTOR ATTORNEY H. G. LUBSZYNSKI ELECTRON EMISSIVE CATHODE Filed Dec. 27, 1946 Aug. 7, 1951 .1 III llull i. flllllllfltlfflilllfl: I III'IIIIIIII Patented Aug. 7, 1951 ELECTRON EMISSIVE CATHODE Hans Gerhard LllbSZYllSi-Zi,,NbtthWd, England, assignor to Electric & Musical Industries Lime x, England, a company of ited, Hayes, Middlese Great Britain Application December 27, 1946, Serial No. 718,668 In Great Britain October 10, 1940 a Section 1, Public Law 690, August 8, 1946 Patent expires October 10, 1960 4 Claims.

The present invention relates to electron discharge devices such as a light transformer which includes a cathode comprising an electrically conducting base having an electron emissive coating, said base having contiguous raised and unraisedportions.

In many electron discharge devices used in or in connection with pictude transmissionor television, photo-cathodes are provided having. an emitting coating formed upon a base of conductive material to provide an electrical connection to the photo-cathode. For example, a light transformer may comprise such a photo-cathode disposed parallel to and spaced from a fluorescent screen. In operation the screen is maintained at a positive potential with respect to said base, and an optical image is focussed on to the photocathode and causes emission therefrom of photoelectrons, which are projected on to the fluorescent screen to form a visible image. In some cases, particularly where the photo-cathode and the fluorescent screen are in close juxtaposition, for example, only separated by a distance of the order of a millimetre, an optical image is projected on the photo-cathode through the conducting base. In cases where the image is projected through the base the latter must be thin enough to be transparent and is usually deposited on an insulating support of mica or other suitable material, for example, by evaporating metal such as silver on to the support, and in order to secure good contact between the base and the lead or leads to the photo-cathode, the base is thickened at its peripheral portion where it is not required, to be transparent. This thickening maybe effected for example, by masking the portion of the conductive base which is required to remain transparent, and evaporating further conductive material, usually silver, on to the portion where the base is to be thickened. Usually, the thickened portion of the base forms a raised annular portion surrounding the unraised central portion.

In operating light transformers having photocathodes with conducting bases having an annular raised portion of the kind described, it has been found that unwanted electron emission takes place from the region of the junction between the unraised and raised portions of the base. Thus, even when the electron-emissive surface was not illuminated it was found that a bright ring was produced on the associated fluorescent screen due to emission from the boundary region referred to. This is particularly so where the photo-cathode and screen are in close juxtaposition since, even when operating 2 the device with a small potential diiierence between the cathode and screen, a very high field gradient is established owing to the close juxtaposition. Such emission is undesirable because, apart from causing undesired illumination of the fluorescent screen, it reduces the contrast of the picture reproduced on the fluorescent screen.

The object of the present invention is to provide an electron discharge device of the kind set forth and an electric circuit arrangement including such a device wherein unwanted emission of electrons which would occur at the junction between the raised and unraised portions of the conducting base of the cathode is eliminated.

In accordance with the present invention, there is provided an electric circuit arrangement including an electron discharge device having a cathode comprising an electrically conducting base which has an electron-emission coating, said base having contiguous raised and unraised portions, said raised portion declining gradually in the neighbourhood of the junction between said portions down to said unraised portion, and wherein the device is operated with such field intensity at said cathode that in the absence of the gradual decline between said raised and unraised portions excessive concentration of electric field would occur at the junction between said portions giving rise to unwanted emission of electrons. The invention further tron discharge device including a cathode of extended area having an electrically conducting base having an electron-emitting coating facing a member which is substantially coextensive with said cathode for receiving electrons emitted from said surface of said cathode and disposed in close juxtaposition tosaid cathode, said base having contiguous raised and unraised portions, said raised portion declining gradually in the neighbourhoodv of the junction between said portions down to said unraised portion so that, in operation, excessive concentration of the electric field between said cathode and said screen giving rise to unwanted emission of electrons which would occur in the absence of a gradual decline at the junction between said portions is avoided.

In a preferred method of carrying the invention into practice, a method of forming the cathode of the device is employed'comprising the step of forming the raised portion of said base by evaporating material on to said base while interposing a mask in front of said base to define said unraised portion, said mask being spaced from said base so as to permit said material to creep provides an elec- 3 under the edges oi the mask whereby said gradual decline is formed.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with ref-- erence to the accompanying drawing, in which- Figure 1 is a diagram illustrating the disposition of the cathode and electron-receiving screen in a device in which unwanted emission of electrons occurs, and

Figure 2 is a diagram similar to Figure l. of a device in accordance with the present invention.

Figure 3 is a, showing of the apparatus used for forming an electrode in accordance with the invention.

Each figure of the drawing shows in section an evacuated envelope in enclosing a mica sheet I on which the conductive base for the photocathode is deposited as indicated at 2. The conductive base 2 may comprise a layer of silver evaporated or otherwise coated on to the mica. but sufliciently thin to be transparent. The silver may be put down by evaporation from a heatedfilament l4 (Figure 3), as is well known in the art. In order to afford good contact to a lead, such as 3, the periphery of the base 2 is thickened as shown at 4. This thickening may be efiected by applying a mask 12 over the part of the base 2 required to remain transparent and evaporatingadditional silver from evaporator 14 past the edge of the mask so as to form a raised annular portion as represented at 4, surrounding the unraisedcentral part of the base 2. If the mask i2 is placed close against the base, the raised annular portion 4 is formed with a sharp inner edge at its junction with the unraised portionas indicated at 5 in Figure 1. when the photo-cathode is in use, for example, in close juxtaposition with an anode electrode comprising a fluorescent screen shown at 6 in Figure 1, lines of electric force are developed between the two screens in the manner indicated by the dotted lines in Figure 1, a concentration of the lines of force I occurring at the sharp corner formed at the edge 5, giving rise to unwanted electron emission from the parts of the base 2 in the vicinity of the edge 5, even when the photocathode is not illuminated. In order to overcome this drawback, the annular portion 4 is formed or treated in such manner as to form a bevel at the boundary between the raised annular portion and the unraised central portion of the base. as indicated at 1 in Figure 2. Thus, the raised portion will decline gradually down to the unraised portion, and consequently, the concentration of lines of force occurring at the edge I of Figure 1 is avoided and the unwanted electron emission will not take place.

The bevelled edge 1 may be formed by spacing the mask i2 away from the base to protect the unraised central portion of the base 2 away from the base while forming the raised annular portion 4. In this way a certain amount of evaporated material will be allowed to creep under the edges of the mask and the desired form of edge will be obtained. The annular portion 4 of Fig- .4 are 1 may alternatively be bevelled mechanically to produce the desired shape.

While the invention has been described in detail in connection with a light transformer having a fluorescent screen arranged in close juxtaposition with aphoto-cathode, it will be understood that the invention is also applicable iircsses where the photo-cathode and the screen on to which the photo-electrons are projected are remote from each other and electron focussing means are employed for focussing the electron beam from the cathode on the screen.

What I claim is:

1. An electron discharge device including a cathode comprising an electrically conducting base and an electron-emissive coating on said base, an anode electrode including a fluorescent screen closely spaced from said base, said base having contiguous raised and unraised portions, said raised portion declining gradually in the neighborhood of the junction between said portions down to said unraised portion to prevent excessive concentration of electric held at the junction between said portions resulting in unwanted emission of electrons between said cathode and said anode screen.

2. An electron discharge device including, a cathode of extended area including an electrically conducting base and an electron-emitting coating on said base, a member including a fluorescent screen substantially coextensive with said cathode and spaced in the order of a millimeter from said cathode coating, said base facing said anode screen having contiguous raised and unraised portions, said raised portion declining gradually in the neighbourhood of the junction between said portions down to said unraised portion to prevent excessive concentration of the electric field between said cathode and said fluorescent screen giving rise to unwanted emission of electrons from said cathode.

3. An electron discharge device according to claim 2, wherein said cathode includes a photoemissive surface and said unraised portion of said base is transparent.

4. A method of forming a cathode electrode on a supporting base, the method comprising the steps of, covering one surface of said base by a mask, spacing said mask from said base surface, evaporating material onto said surface of the base not covered by said mask to form a raised portion of said base, continuing the evaporation of said material until a decline is formed between the said raised portion and said base by the material collecting on said base under the edges of the spaced mask.

HANS GERHARD LUBSZYNSKI.

REFERENCES CITED The following references are of record in the file of this patent: 1

UNITED STATES PATENTS 

